1. Field of the Invention
The present invention relates to activated carbon and methods for preparing same. Particularly, this invention relates to new carbons useful in vapor adsorption and methods for their production. More particularly, this invention relates to activated carbon derived from lignocellulosic material prepared by chemical activation and agglomeration to produce carbon of high density and high activity.
2. Description of the Prior Art
Activated carbon is a microcrystalline, nongraphitic form of carbon which has been processed to increase internal porosity. Activated carbons are characterized by a large specific surface area typically in the range of 500-2500 m.sup.2 /g, which permits its industrial use in the purification of liquids and gases by the adsorption of gases and vapors from gases and of dissolved or dispersed substances from liquids. Commercial grades of activated carbon are designated as either gas-phase or liquid-phase adsorbents. Liquid-phase carbons generally may be powdered, granular, or shaped; gas-phase, vapor-adsorbent carbons are hard granules or hard, relatively dust-free shaped pellets.
Generally, the larger the surface area of the activated carbon, the greater its adsorption capacity. The available surface area of activated carbon is dependent on its pore volume. Since the surface area per unit volume decreases as individual pore size increases, large surface area is maximized by maximizing the number of pores of very small dimensions and/or minimizing the number of pores of very large dimensions. Pore sizes are defined herein as micropores (pore width &lt;1.8 nm), mesopores (pore width=1.8-50 nm), and macropores (pore width&lt;50 nm). Micropores and mesopores contribute to the vapor adsorptive capacity of the activated carbon; whereas, the macropores reduce the density and can be detrimental to the vapor adsorbent effectiveness of the activated carbon, on a carbon volume basis. The Adsorption capacity and rate of adsorption depend to a large extent upon the internal surface area and pore size distribution. Conventional chemically activated lignocellulose-based carbons generally exhibit macroporosity (macropore volume) of greater than 20% of the carbon particle total volume. Gas-phase activated carbon macroporosity of less than 20% of the carbon particle volume would be desirable. Likewise, a high percentage of mesoporosity (i.e., above 50% of total particle volume) is desirable.
Commercial activated carbon has been made from material of plant origin, such as hardwood and softwood, corncobs, kelp, coffee beans, rice hulls, fruit pits, nutshells, and wastes such as bagasse and lignin. Activated carbon also has been made from peat, lignite, soft and hard coals, tars and pitches, asphalt, petroleum residues, and carbon black.
Activation of the raw material is accomplished by one of two distinct processes: (1) chemical activation, or (2) thermal activation. The effective porosity of activated carbon produced by thermal activation is the result of gasification of the carbon at relatively high temperatures (after an initial carbonization of the raw material), but the porosity of chemically activated products generally is created by chemical dehydration/condensation reactions occurring at significantly lower temperatures.
Chemical activation typically is carried out commercially in a single kiln. The carbonaceous material precursor is impregnated with a chemical activation agent, and the blend is heated to a temperature of 450.degree.-700.degree. C. Chemical activation agents reduce the formation of tar and other by-products, thereby increasing yield.
A "hard active carbon of high adsorptive power in the shaped or moulded state" is taught in U.S. Pat. No. 2,083,303 to be prepared by impregnating pulverized organic raw material, such as "sawdust, peat, lignite or the like" with "known activating agents, such as zinc chloride or phosphoric acid" and heated to 100.degree.-200.degree. C. for one to one and a half hours producing a partially carbonized state wherein the material is somewhat plastic. Without reducing the temperature, the material is molded under pressure to a desired shape. The shaped material then is activated in a rotary activating retort and brought to a temperature of 450.degree.-600.degree. C. for about four hours.
Similarly, U.S. Pat. No. 2,508,474 teaches a gas mask activated carbon to be prepared by impregnating low density cellulosic material, such as finely divided wood in the form of wood shavings or sawdust, with concentrated zinc chloride, and heating to 120.degree.-145.degree. C. while agitating for not less than fifty minutes. The reacted mass then is compacted into "forms of appreciable size;" said forms are dried at 160.degree.-300.degree. C.; the dried forms are crushed into granular particles; the granules are calcined at 675.degree.-725.degree. C.; and, after leaching out of the particles a greater portion of residual zinc chloride, recalcining the activated carbon product at 1000.degree.-1100.degree. C. for at least thirty minutes.
These representative techniques have produced activated carbon of adequate activity and density for many gas-phase applications, especially for purification and separation of gases as in industrial gas streams, in odor removal in air conditioning systems, and in gas masks. However, older technology gas-phase activated carbons have not proven entirely satisfactory in some applications for recovery (not just removal) of organic vapors which involves adsorption onto the carbon surface followed by desorption from the carbon for recapture. In fact, due to environmental concerns and regulatory mandates, one of the largest single applications for gas-phase carbon is in gasoline vapor emission control canisters on automobiles. Evaporative emissions vented from both fuel tank and carburetor are captured by activated carbon.
Fuel vapors, vented when the fuel tank or carburetor is heated, are captured in canisters generally containing from 0.5 to 2 liters of activated carbon. Regeneration of the carbon is accomplished by using intake manifold vacuum to draw air through the canister. The air carries desorbed vapor into the engine where it is burned during normal operation. An evaporative emission control carbon should have suitable hardness, a high vapor working capacity, and a high saturation capacity. The working capacity of a carbon for gasoline vapor is determined by the adsorption-desorption temperature differential, by the volume of purge air which flows through the carbon canister, and by the extent to which irreversibly adsorbed, high molecular weight gasoline components accumulate on the carbon.
Because of various economic considerations and space limitations in placing the carbon canister on-board a vehicle, this particular application of granular or shaped activated carbon requires higher activity and higher density properties than typically produced by the older technology noted. One method to control product density is taught by published European Patent Application 0 423 967 A2. The applicants note "a number of problems inherent in the use of wood as a raw material to produce directly a chemically activated pelletised form," claiming it to be "impossible to produce a high density activated carbon from a wood flour material" for lack of sufficient natural binding agent. An improved product (of substantially increased density) is claimed by use of, as a starting material, a "young carbonaceous vegetable product" having a "high concentration of natural binding agent." Such materials include nut shell, fruit stone and kernel, and in particular olive stone, almond shell, and coconut shell.
Also, U.S. Pat. No. 5,039,651 teaches densification of activated carbon product from cellulose materials including coconut shells, wood chips, and sawdust by pressing after initially heating to a relatively low temperature, followed by extrusion and calcination. Yet, with this improved processing the patentees could produce only carbons that were measured to have a volumetric working capacity (in terms of butane working capacity, or BWC) of up to 12.3 g/100 cm.sup.3, although BWC values up to 15 g/100 cm.sup.3 are claimed.
These prior art gas-phase carbons may have been satisfactory for limited volumes of vapors emitted from the carburetor and fuel tank. Because of impending environmental regulations requiring capture of greater amounts of fuel vapor emissions, it is anticipated that the volume of these additional vapors, combined with the space limitations and economic considerations which limit expansion of the size of canister systems, will require activated carbons with higher densities, higher activities, and higher volumetric working capacities than disclosed by the prior art (e.g., BWC&gt;15 g/100 cm.sup.3).
Recently, co-pending (and commonly assigned) application Serial No. 07/853,133 described activated carbons of high activity and relatively high density suitable for solvent and vapor capture and recovery prepared by chemically activating carbonaceous material fragments (i.e., "discrete particles"), heat plasticizing the particles to begin transition to thermoset, densifying the particles by mechanical shaping (in a spheronizer), further heating the shaped particles to thermoset, and further heating the thermoset shaped particles to 425.degree.-650.degree. C. Unfortunately, the spheronizing equipment limitations related to such process restrict capacity to below commercial production levels. It is an object of this invention to provide a novel alternative chemical activation process for producing high activity, high density gas-phase activated carbons on a commercial scale. It is a further object of this invention to provide a chemical activation and agglomeration process for producing high activity gas-phase activated carbons without sacrificing density.